Visual prostheses have been developed to restore or improve vision within blind or partially blind patients. A visual prosthesis commonly includes an implantable component having an electrode array, situated on or in a substrate, for placement in the eye on or near retinal nerve cells. Electrical signals are transmitted via the electrodes to the retinal nerve cells, triggering a perception of light within the patient's brain. The prosthesis can therefore restore or improve vision to patients whose retinal photoreceptors have become dysfunctional.
Commonly, a visual prosthesis is used in conjunction with a video camera. A stream of images detected by the camera is converted into digital signals by an image processor and transmitted in ‘real time’ to an electrode interface unit. The electrode interface unit is connected to the electrode array via a plurality of conductors and decodes the signals and stimulates the electrodes in accordance with the detected images.
Conventional electrode stimulation techniques apply current to one electrode at a time, and seek to switch between application of current to electrodes of the array fast enough to cause flicker-free vision, either using single or multiple current sources. For epiretinal implants with 16-site electrode arrays it has been shown that fast sequential application of current to electrodes, using a raster scanning process for example, can elicit the perception of continuous elements. Typically, each electrode represents a single “pixel” in a coarse array of pixels derived from the image. Pixel-based techniques such as this are disclosed in US 2008/0058897 A1 and US 2008/0046030 A1, for example.
It is desirable to provide the perception of increased image quality to the patient through use of larger electrode arrays representing images with a greater number and finer spacing of pixels. However, when using larger electrode arrays, including arrays of hundreds or thousands of electrodes, conventional pixel-based techniques have been found to have significant engineering and technical constraints. For instance, when sequential application of current to electrodes is employed in larger arrays, the stimulation may not be fast enough to provide flicker-free vision to the patient.
To solve this problem, simultaneous application of current to multiple electrodes has been performed. However, “cross-talk” or current interactions between the electrodes has been found problematic as it leads to enlarging of the various tissue areas stimulated by the electrodes, blurring stimulation between these areas and thus the resultant image perceived by the patient. In light of this, it has been proposed in US 2006/0241753 A1 to apply current to hexagonal patterns of electrodes using multiple current sources. Hexagonal ‘guard’ rings of electrodes are created, which each surround a central electrode, and act as current return electrodes, keeping the current applied to the central electrode focussed. By using multiple current sources, current can be applied to many such hexagonal patterns simultaneously. However, this technique requires specialised hardware with multiple current sources, increasing cost and component sizes. The use of hexagonal guard rings also increases thresholds for electrical stimulation and therefore increases the total power consumption of a device when compared to devices having simpler stimulus patterns, such as monopolar stimulus patterns.
Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present invention as it existed before the priority date of each claim of this application.
Throughout this specification the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.
Throughout this specification the term “visual prosthesis” is used to denote apparatus for restoring or improving visual function of a patient, and will be understood to cover devices otherwise known as bionic eyes, artificial eyes, retinal prostheses and retinal stimulators or similar.